1. Field of the Invention
This invention relates to the field of electrical contactors for motor starters and similar devices, and in particular to an electronic interlock that senses the open or closed condition of the contactor by monitoring the inductance of the contactor coil, the inductance changing as a function of the air gap between the armature and the magnet.
2. Prior Art
Electromagnetic contactors having one or more sets of contacts which are opened and closed by voltage applied to a coil are useful for various switching and control functions. A contactor usually has a magnetic circuit which includes a fixed magnet and a movable magnet or armature, with an air gap between them when the contactor is opened. An electromagnetic coil is controllable upon command to interact with a source of voltage which can be coupled across the main contacts of the contactor for electromagnetically accelerating the armature towards the fixed magnet, thus reducing the air gap.
The armature carries a set of bridging contacts, operable to electrically connect fixed contacts, mounted in the contactor case, as the magnetic circuit is energized and the armature is moved. The load and the voltage source are usually connected to the fixed contacts and become interconnected with one another as the bridging contacts make with the fixed contacts.
As the armature is accelerated towards the magnet, it is opposed by two spring forces. The first spring force is due to a kickout spring which is subsequently used to disengage the contacts by moving the armature in the opposite direction when the power applied to the coil has been removed. This occurs as the contacts are opened. The other spring force is due to a contact spring which begins to compress as the bridging contacts abut the fixed contacts, but while the armature is still moving towards the fixed magnet as the air gap is reduced to zero.
Unlike a simple circuit breaker, which opens contacts in overcurrent conditions and must be manually reset, a contactor may be arranged to open and close contacts in various ways, sometimes repeatedly, for example to start, stop, coast or reverse a motor. Contactors can be combined with various overload protection means, in which case the contactor is typically called a motor controller. Single phase and multiphase contactor switching arrangements can be used, and high current switching capability can be provided.
There are numerous possibilities for specific applications of contactors. With a bidirectional motor, for example, two contactors may be coupled to the motor circuit, one for establishing contacts for forward rotation and the other for reverse. Other possibilities include varying the connections to a motor for starting or stopping in sequential steps, connecting the motor as an autotransformer, switching between wye and delta connections, etc. In order to control or coordinate operation of one or more contactors coupled to a control apparatus, or to a manually operated switching means, it is often desirable to provide a signal which indicates the present status of the contactor, i.e., whether the contacts are open or closed. This signal can be used by the control circuits for switching between respective control states. It would be possible to use, the switched voltage to provide such a status indication. However, this is not desirable for a number of reasons. The voltage is very noisy due to the current variations caused by contact bounce and by the typically inductive nature of the load. The state of the contacts may need to be determined before a voltage is available at the contacts, e.g., before operating a switching means more proximally coupled to the power mains. On the other hand, coupling the contact voltage to an external sensing device requires additional parts. For all these reasons, it would be desirable to provide a different form of interlock.
Apart from signals which may be coupled to external control circuits to govern operation of a system including an electromagnetic contactor, internal control circuits for contactors are known and used for various purposes. U.S. Pat. No. 4,893,102--Bauer teaches a contactor apparatus including a microprocessor controller operable to vary the power applied to the contactor coil during a closing stroke, to accelerate the armature at high power during an initial phase of closing, then to coast and finally to maintain contact, at reduced power. This arrangement reduces mechanical shock, contact bounce and other adverse effects of accelerating the armature at equal power over the stroke. The power applied to the coil is varied by a timing technique wherein a triac is triggered at progressively later times during the alternating current half cycles so as to apply progressively less current to the coil.
An objective according to Bauer is to control the velocity of the armature by reducing the coil drive current to a hold level after applying only sufficient acceleration for the armature to complete the stroke. The velocity of the armature thus slows to zero just as the air gap reaches zero and the remaining coil drive current holds the contacts closed. However, it is difficult to set this relationship exactly, or once the relationship is set to assume that it will not change over time. It would be desirable in a controller according to Bauer to sense when the air gap has reached zero. This would save power by providing feedback to the controller as to the particular amount of acceleration which is needed to just overcome the kickout spring and the contact spring.
U.S. Pat. No. 4,819,118--Mueller et al also teaches a microprocessor controller for a contactor system. Two contactors each have their own controllers, and are arranged to apply power to a reversing motor for rotation in opposite directions. The two controllers are coupled in communication and either can cause both contactors to trip in the event of thermal overloading of the motor. Current supplied to the load from each of the two contactors is monitored using analog to digital converters. The respective controller's microprocessor samples the output of the analog to digital converter and develops an estimation of the heat accumulated by the motor in its reversing operation. In this manner the current applied through both contactors is used to determine heating in the load, rather than only the current applied through the contactor which happens to be active.
It is an object of the present invention to sense the open/closed status of- an electromagnetic contactor by monitoring for the change in inductance of the coil circuit which occurs as a function of the air gap, or lack of an air gap, between the armature and the coil of the magnetic circuit.
It is another object of the invention to apply the current and voltage level sensing apparatus of a microprocessor controlled electromagnetic contactors to collect sufficient information to detect the change in the inductance of the magnetic circuit.
It is a further object of the invention to improve the operation of a microprocessor controller which switches current to a contactor coil in timed partial half cycles by sensing the change in the phase angle of switching which occurs between the open and closed states of the contactor.
These and other objects are accomplished in an electrical contactor having first and second contacts movably mounted to engage for achieving continuity in an electrical circuit, via an electromagnet and armature defining a magnetic circuit with an air gap that is closed in a first position of the contactor, normally when the contacts are made, and open in a second position of the contactor. A controller switches an alternating current voltage to the coil of the electromagnet during a timed portion of each AC half cycle, and senses the current level in the coil in a feedback loop. The controller adjusts the voltage-on time to achieve a predetermined average current as needed for accelerating the armature or coasting during a closing operation, for holding the armature in place when closed, etc. In order to sense whether the contactor is presently open or closed, the controller monitors and stores the phase angle between the previous voltage zero crossing and the time of voltage turn-on. When the inductance of the magnetic circuit including the electromagnet and armature changes rapidly due to opening or closing of the air gap between them, the controller detects a corresponding variation in the phase angle. The controller is preferably a microprocessor, programmed to normalize the phase angle over a range of coil drive voltages. The microprocessors of a number of such contactors can communicate in order to effect coordinated operations.
The microprocessor can test the inductance of the magnetic circuit including the coil and armature during a known status of the contactor. For example, after initiating a closing operation by applying wide voltage pulses to the coil, the microprocessor can output a train of shorter test pulses, e.g., of sufficient width to hold the armature once the contacts are closed, monitoring and storing the phase angle of the time at which the voltage must be switched on to maintain a holding current level. As the air gap closes, the inductance and the phase angle change, whereupon the microprocessor outputs an appropriate signal indicating a closed status or otherwise branches in its control routine in view of the change in status.
Similarly, during an opening operation the microprocessor ceases switching voltage to the electromagnet for sufficient time to allow the kickout spring and contact spring to exert a force which will accelerate the armature open, then outputs a series of pulses, again maintaining a given current level in the electromagnet and monitoring for the change in voltage switching phase angle that indicates opening of the air gap and a drop in inductance of the magnetic circuit. The same operation can also detect sudden opening (i.e., opening due to mechanical shock or the like) during an ongoing close-and-hold operation.
The invention facilitates coordinated operation of a number of contactors, each having a local control microprocessor operable to detect the status of the local contactor unit, and each being coupled in data communication with one or more others. An example is a coordinated motor reversing operation in a contactor arrangement having a first contactor for operating the motor in a forward mode and a second contactor for reverse. The invention is readily applicable to a contactor of the type having a microprocessor or similar controller to vary the current level in the electromagnetic coil to effect soft-close operations in which the level of energy applied to pull in the armature is minimized to the amount needed to overcome the kickout spring and contact spring force.